This is a huge bed extension for the Mini!
It gives you 13" of y travel, but you have to ditch the heated bed or make a custom one. You can use any length rod to make it any size.
Some flat substrate for a new print bed
m4 screws (somewhere between 25mm-35mm would be best I believe)
2x406mmx8mm linear bearing rods
some gt2 belt
One small Zip Tie
It's not perfect but I included the fusion file so you can modify as you like! Modify your slicer to the larger work envelope, and push bed to within it's normal travel range before you start a print so it will home properly. No other mods required.